1. Field of the Invention
This invention relates to a super-resolution optical element having a plurality of divided portions for use in an optical disk apparatus and an image forming apparatus, and more particularly, to a method reducing a beam spot size and for enhancing light-intensity of a beam spot with the super-resolution optical element.
2. Description of the Related Art
In an optical disk apparatus, information is recorded in an optical disk (recording medium) and information recorded in the disk is read therefrom, by focusing a light beam with a beam waist on the recording surface of the disk. Though it has been required these days to increase the recording density (i.e., the amount of information which can be stored in the optical disk), the recording density is limited by the beam spot size of the beam waist of a light beam. That is, so as to increase the recording density, it is necessary to minimize the beam spot size.
Further, in an image forming apparatus (printer apparatus), a light beam with a beam waist is radiated onto a photoconductor (recording medium), and a latent image corresponding to the light beam is formed on the photoconductor, resulting in a visual image. The gradation, resolving power and half tone density of the visual image are determined by the beam spot size of the beam waist of a light beam used to form a latent image. Hence, it is required to use a light beam of a small beam spot size in order to visualize the gradation, resolving power, and half tone density in more detail.
In an optical microscope, it is know that the higher the magnifying power, the lower the light intensity of the beam spot. This being so, in general, the intensity of light beams emitted from a light source (or voltage to be applied to a light source driver) must be high. However, to emit light beams of a high intensity, a large-size light source is needed, which will generate a great amount of heat.
The beam spot size of a light beam radiated onto an optical disk Rm is reduced up to a limit value determined by the refractive index of an objective lens employed. Specifically, the diameter "W" of the beam spot is limited by the aperture diameter "A" (A=2F.times.NA) of the lens, where "F" represents the focal distance of the lens, and "NA" the numerical aperture.
This being so, as is shown in FIG. 53, the minimum pitch at which pits (i.e., data items) can be aligned in an optical disk is generally larger than the diameter "W" of the beam spot (in practice, however, a light beam of w=1.25.times.W is radiated since its beam spot has a certain energy intensity distribution).
To overcome this, so-called super-resolution methods have been proposed which can provide a beam spot "W" of a diameter smaller than the above-described limit value determined by the refractive index of the objective lens.
For example, there are known a center mask method (M. Born and E. Wolf: Principles of Optics, Pergamon Press Ltd. Oxford, 1975) for masking a central portion of an aperture area (which exists in an optical element such as lens means and through which light beams can pass), and a phase-shift method (J. E. Wilkins, Jr.: J. Oct. Soc. Am., 40 (1950) 22) for concentrically dividing the beam spot of a light beam into two areas, and shifting by 180.degree. the phases of light beams passing through the areas, respectively.
However, it is known that in the center mask method and phase-shift method, the smaller the beam spot size, the greatly lower the intensity (center peak intensity) of the center beam spot and the higher the intensity of the sidelobes. Here, note that in the super-resolution method, since the beam spot has a plurality of sidelobes, i.e., fringes, the center beam spot is actually a coalescence point located in the center of the sidelobes.
U.S. Pat. No. 5,121,378 (filed Jun. 9, 1992) discloses an optical head device employing a super-resolution element.
In the optical disk apparatus, when the intensity of the center beam spot is low, a recording error may occur at the time of recording information in the recording medium, and a reading error may occur at the time of reading information out of the medium. Further, under the same condition, characters may become blurred or solid lines may be broken in the image forming apparatus, and satisfactory observation may not be performed in the microscope since the illumination intensity of an object image (including a virtual image) will be reduced.
Thus, in many cases, the output of a light source must be increased so as to obtain a sufficient intensity of the center beam spot. To increase the output of a light source in the disk apparatus, the light source must be made large, thereby inevitably making heavy an optical head unit in which the light source is mounted. Where the optical head unit is heavy, an access speed may be low. Further, in the image forming apparatus, decay in the light to be radiated onto the recording medium may be increased, and the half tone density and gradation of an image may become imbalanced. Moreover, in the microscope, there may occur undesirable problems in restraining an increase in temperature or in reducing its size.
On the other hand, increasing the intensity of the sidelobes may cause cross talk in the disk apparatus at the time of reading information out of the recording medium. In the image forming apparatus, under the same condition, a ghost image may occur around a real image or the width of a line may be enlarged. Further, in the microscope, the contrast of an object (or virtual) image may be degraded under the same condition.